Don’t Sweat It: Comfort Conditioning Can Increase Thermal Comfort, Decrease Energy Use

By Mary Kate McGowan, Associate Editor, News
From ASHRAE Journal Newsletter, July 10, 2018

It's not the heat. It's the humidity. It could also be about sweat, and the building might matter too.

And mechanically air conditioning a whole building might not be the best solution to achieve occupant thermal comfort, said Robert Bean, Member ASHRAE, and member of ASHRAE’s Residential Buildings Committee.

In hot and humid climates, controlled humidity, air temperature, mean radiant, air speed and personal factors affect people’s perceptions of their thermal comfort, said Bean.

To achieve a comfortable environment, building sciences, mechanical systems and energy must be taken into consideration, said Bean, who is also a project committee voting member on ASHRAE Standing Standard Project Committee 55, Thermal Environmental Conditions for Human Occupancy.

A combination of using dehumidifiers, ceiling and desk fans, improving building enclosures and wearing reduced clothing, comfort conditioning can increase occupant thermal comfort and use less energy than an air conditioning system, he said.

This is a trend that will continue to evolve with required improvements in architectural designs and enclosures, Bean said.

The Science Behind Sweating

As part of the “How Can Net Zero Energy Goals, Low Sensible Loads and Ventilation Work in a Hot Humid Climate?” seminar during the 2018 ASHRAE Annual Conference in Houston, Bean addressed how human physiology relates to hot, humid climates and the thermal regulatory processes the people go through as they experience these challenging climates.

As space temperature and humidity increase, the body’s mechanisms of controlling its core temperature becomes muted, he said.

“When we sweat, we release not only sensible but latent heat,” he said. “It’s really important that when we get into these environments where the temperature and humidity are high that we control it and try to reduce it.”

When it comes to thermal comfort, engineers have to know and think in terms of what is the humidity in the space and its vapor pressure relative to the body, Bean said.

If the vapor pressure in the space is higher than the body, the body loses its ability to shed heat via evaporation. As long as a space’s vapor pressure is lower than the body, the body can get rid of some excess heat via the latent cooling process.

Solutions and Challenges

In some indoor conditions, keeping humidity levels under control with increased air speeds and reduced clothing enables occupants to shed excess body heat. This leads to perceptions of a better indoor environment, he said.

Unfortunately, the default solution to achieving cooling thermal comfort is mechanically air conditioning the whole building. This requires larger than necessary compressors and fans, which have high electrical loads, Bean said.

The problem, he said, is the disconnect between better enclosures, reducing moisture and providing conditions for comfort using whole building air conditioners.

As buildings improve, there are insufficient sensible loads even on the smallest off-the-shelf cooling coil. As a result the systems on/off cycle rate increases. This re-introduces moisture from the wet coil back into the air stream increasing the relative humidity in the space, according to Bean.

Trying to solve the problem with oversized equipment and setting the thermostat lower only exacerbates the problem, he said.

A better first solution is to still reduce sensible loads with improved building enclosures, he said. These tighter buildings with reduced thermal bridging and lower U values employ better windows, external shading and lower WWR that combined slow down and keep the short and longwave radiation off and out of the building.

Humidity generated by occupants and brought in through ventilation can then be reduced using small standalone dedicated dehumidifiers.

In combination with ceiling and desk fans and reduced clothing, comfort conditioning is more easily achieved with a much lower energy penalty.

“We’re able to provide comfort conditions without having to run whole home air conditioners,” Bean said.

Controlled Humidity

Controlled humidity works to achieve occupant thermal comfort, but not in all cases, he said.

When the air and mean radiant temperature are controlled to 72°F (22.2°C), and air speed is low—about 20 fpm (0.1 m/s) with occupants lightly clothed in a low met rate activity such as reading—they will feel slightly cool regardless of changes to the humidity.

This is because at the stated conditions, cooling is provided primarily through radiation and convection. In fact, that space would not be in compliance with Standard 55, Thermal Environmental Conditions For Human Occupancy, as it would be perceived to be too cool for some people, he said.

He added that even though a higher humidity will have little effect on comfort, the humidity should not climb for reasons related to viruses, bacteria and molds, hydrolysis and IAQ. Bean said IAQ includes perceptions with odors and the effect high moisture has on hygroscopic materials such as woods.

Bean provided another example where the humidity is allowed to climb to 60% with the air temperature controlled to say 68°F (20°C), but the mean radiant is at 76°F( 24.4°C) due to a poor enclosure.

If the air speed around an occupant walking around is low and that person is wearing formal business attire, the space would also not be in compliance with the standard as it would be perceived to be too warm.

Only when the humidity is dropped to 35% would occupants perceive the space to be comfortable.

The reasons is the decrease in space vapor pressure relative to the body encourages shedding of heat via evaporation, Bean said.

The problem with this lower level of humidity is increased airborne transmission of viruses and bacteria as shown by recent research work in hospitals, and there is also the energy penalty to consider.

In such cases it would be better to control to say 50% humidity but elevate the air speed while encouraging a relaxed clothing policy, he said.

Recommendations

Bean recommends for controlled humidity to hover between 35-55% +/- 5%.

“When you control humidity to a lower value, it takes more energy, promotes microbial transmission and can have some dimensional effect on hygroscopic materials. If you allow the humidity to climb up, it takes less energy but promotes hydrolysis (VOC emissions) and again can have some dimensional effect on hygroscopic materials,” he said.

The higher ranges increase the probability of condensation risks on cooled surfaces such as ducts and pipes which leads to saturation, dripping and increases in moisture on adjacent surfaces which enables growth of molds.

“The nice place is that middle ground,” he said.

To do this with the least amount of energy, the sensible loads on and in the building need to be reduced.

There is no reason to not get loads down below 10 Btu/hr/ft2 or 1200 ft2 /ton with affordable design solutions to the enclosure, he said.

Then there is the moisture load, and there is no better solution than the small dedicated dehumidifiers, Bean said. Such units are available as standalone floor mount, or wall cavity mount models or models that can be incorporated into an HRV/ERV.

Dedicated ventilation, standalone or incorporated dehumidification with independent space heating and cooling is something Bean has been doing since 1983. This is not a new approach he said.

“What is new...is old but better building technology is being adopted by the broader market and the consequences are directing them to what we knew over 30 years ago,” Bean said.